Weather Watch

Sonic booms are force of man — and nature

PGEweather@pge.comJanuary 12, 2013 

On a clear day in 1991 off the coast of California I landed with my crew in our Navy H-3 Sea King helicopter aboard the aircraft carrier USS Nimitz after flying on plane guard. On plane guard you fly in perpetual orbits on the starboard side of the carrier, waiting patiently to rescue survivors of aircraft mishaps or those unfortunate flight deck crew members blown overboard.

After we landed, I hopped out the helicopter’s cargo door onto the Nimitz’s flight deck. I signaled the enlisted aircraft signalman standing in front of the helicopter for permission to walk under the spinning rotors. As I walked under the rotor arc, the signalman started to point to the sky. It was a hand signal that I had never seen before and I had no idea what he was attempting to communicate. A moment later, I was knocked to the deck by an intense pressure wave. After seeing stars, I looked up and saw two F-14 Tomcats’ fighter jets in full afterburner racing straight up into the sky at unfathomable speeds.

On a cool day last December, an F-22 Raptor along with an F-16 Falcon flew off the coast and produced sonic booms that rocked San Luis Obispo County. People either heard a sonic boom or a loud prolonged scratchy noise, like a rocket taking off from Vandenberg or a meteor skipping across the sky.

Historically along the Central Coast, the space shuttle would single its return to Edwards Air Force Base by a sonic boom. Other natural phenomena produce loud booms. In January 2010, Larry and Judy Cobbs were walking through Dinosaur Caves Park in Pismo Beach when a large section of the coastal bluff collapsed into the ocean in front of them. Their neighbors thought it was an earthquake or sonic boom.

When an object moves faster than the speed of sound, it produces a shock wave that forms a cone of pressurized air that moves across the Earth's surface. The sharp release of pressure is heard as the sonic boom. The speed of sound in air at sea level is about 761 mph, depending on air density. Generally, the higher your altitude, the less dense the air will become and the slower the speed of sound. For example, the speed of sound is about 100-mph slower at 50,000 feet.

The four factors that most affect air density are altitude, atmospheric pressure, air temperature and humidity. Increasing the air temperature will decrease its density but will increase the speed of sound due to the air molecules' increased elasticity. It may seem counter-intuitive, but humid air is lighter, or less dense, than dry air.

In 1947 Chuck Yeager became the first person to fly faster than the speed of sound in the X-1 rocket-powered aircraft. He had the courage to break through the "wall" of intense pressure before achieving supersonic speeds. I had the honor to meet Chuck Yeager while standing next to our helicopter while he toured the USS Carl Vinson. I will never forget how his face lit up when he saw the helicopter’s rescue hoists.

PG&E will offer training courses this year for recently discharged veterans that will provide a career path into the energy industry through the utility’s PowerPathway program. For more information, visit www.pge.com.

If you have any questions or comments about weather or this column, I would love to hear from you. You can also subscribe to my daily weather forecast by emailing me at PGEweather@pge.com.

John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist.

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